How did Ptolemy explain retrograde motion?
How did Ptolemy explain retrograde motion?
Ptolemy's theory of the solar system.
Encyclopædia Britannica, Inc.
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Encyclopædia Britannica, Inc.Ptolemy's theory of the solar system.
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© MinutePhysics (A Britannica Publishing Partner)Speculation about and descriptions of different multiverses.
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© MinutePhysics (A Britannica Publishing Partner)Learn about the distinction between the observable universe and the whole universe.
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© MinutePhysics (A Britannica Publishing Partner)A brief overview of space expansion.
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Encyclopædia Britannica, Inc.Discussion of four attempts to explain the structure of the solar system, from Aristotle to Johannes Kepler.
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Encyclopædia Britannica, Inc.History of the big-bang model.
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© MinutePhysics (A Britannica Publishing Partner)An overview of the Millennium Simulation run by researchers at the Max Planck Institute for Astrophysics in Germany, followed by a tutorial on how to simulate the universe on a home computer.
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Encyclopædia Britannica, Inc.Copernicus's theory of the solar system.
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© MinutePhysics (A Britannica Publishing Partner)Astrophysicist Neil deGrasse Tyson narrating a brief history of the universe, from the big bang to the present.
Transcript
NARRATOR: Aristotle's model of the universe had trouble explaining some planetary phenomena. The most striking of these was retrograde motion. In retrograde motion each planet seems to slow down at times, then move in reverse, or retrograde, before resuming its course. Planets also grow brighter or dimmer as they move through the sky. Aristotle's model could account for neither phenomena very well. The most important solution to this problem was proposed by Claudius Ptolemy in the 3rd century AD. He argued that planets move on two sets of circles, a deferent and an epicycle. This explained retrograde motion while keeping the planets in their circular orbits around the Earth. Where this did not fit, Ptolemy proposed an eccentric. An eccentric orbit had a center different from the Earth and accounted well for changes in a planet's brightness. Ptolemy's last device was the equant. In an equant, a planet speeded up and slowed down but when seen from an off-center point actually appeared to be moving with uniform speed. From Earth, however, the planet's motion was quite irregular.
The Ptolemaic system held ground for centuries until too many discrepancies cried for new solutions.
The Ptolemaic system held ground for centuries until too many discrepancies cried for new solutions.